The present invention concerns generally the drying of veneer sheets to a moisture level suitable for subsequent use of the sheets in a manufactured product such as plywood panels.
Typically, wood veneer is cut from a lathe mounted block with the sheet subsequently being clipped or cut and then dried to a desired moisture content. Typically the panels or sheets are dried by passage through a dryer on a conveyor wherein hot air flows impinge on both sides of the veneer resulting in heating and partial drying of same. Considerable effort must be directed toward the drying of veneer to prevent over-drying which results in a defective manufactured product such as plywood. Conversely, under drying of veneer can also result in a defective finished product. Still further, excessive shrinkage of a panel may result in an over-dried veneer panel. Veneer panels still having excessive moisture content after passage through a dryer must be subjected to a second drying operation at a considerable reduction in the efficiency of a dryer operation. Fires in veneer dryers are not unknown and result from the veneer being heated to the point of combustion and result in costly damage to the dryer and of course the loss of veneer sheets therein. Dryer fires are costly also from the standpoint of down time of a veneer plant for purposes of dryer repair.
Incorporated herein by reference is U.S. Pat. No. 4,193,207 and specifically columns 1, 2, 3 and lines 1-52 of column 4.
U.S. Pat. No. 4,193,207 discloses a veneer dryer having a rotatable frame on which are mounted a circular array of hot presses. The frame is indexed to sequentially locate the hot presses at an on-loading station and an off-loading station. The presses each include a series of grooved heated plates which close toward one another with a veneer panel between each of the plates for contact drying of several panels during closure of the hot press plates. The heated plates dry the veneer by heat transfer and, upon indexing of the dryer frame, each hot press is positioned for off-loading of the sheets or panels upon opening of the press. Drawbacks to the dryer disclosed in the above noted patent include complexity and hence a high capital cost and the modification of plant facilities for installation of same. The utilization of the several hot presses on a frame each with a multitude of heated plates each served by a flexible steam lines would incur high maintenance costs as would the pair of hydraulic cylinders for closing each hot press.
Important objectives of the present veneer dryer include reduced substantially electricity consumption and increasing thermal efficiency in the contact heating of veneer panels; the avoidance of plug ups occurring in existing veneer dryers resulting from the divergence of a panel from the desired path through a dryer; the avoidance of fire resulting in costly damage to and down time of a conventional dryer; the reduction of hydrocarbon particles emitted from dryers; the avoidance of periodically cleaning of a conventional veneer dryer to remove wood fragments and deposits; the provision of a dryer with reduced maintenance costs than that incurred with conventional veneer dryers.
The present invention is embodied within an apparatus for reducing the moisture content of veneer panels by contact with a heated surface of a wheel member.
The present dryer includes a wheel having a heated continuous surface for the reception of veneer sheets or panels. An annular or circular chamber of the wheel receives a heated medium. Provision is made for circulating the medium through the chamber. Retention of the panels on the wheel outer surface includes, in one form of the invention, multiple runs of cable directed in overlying fashion on the veneer. Retention of panels may be achieved otherwise, as for example, such as by a continuous screen. A drive system imparts rotation to the wheel at a speed to achieve the desired duration of panel heating and moisture reduction. The cable runs, or other panel retention components, may be initially utilized to carry the panels, in sequence, into wheel contact at an on-loading site of the apparatus. Pre-heating of such panels, if necessary, is achieved by a pre-heater at the on-loading site. Provision is made for controlling the speed of wheel rotation while additional control means enables the temperature of the heated medium to be varied to accomplish the specific drying tasks. A method of reducing the moisture content in wood veneer panels also constitutes part of the present invention.